APOD: Planetary Nebula Abell 7 (2013 Dec 05)

[Ann]

The reason why an 18,000 K blackbody is generally considered the most blue (and why most people see it that way) is that this is the temperature where you have the most "blue" (450 nm) energy in comparison with longer visual wavelengths. As you get hotter, the energy across the visual spectrum gets flatter, with relatively more energy at longer wavelengths compared to blue (you always have more in total at the short end, it's just that the distribution gets flatter).

Thanks, Chris. That makes sense.

Ann

[Anthony Barreiro]

Why do stars look their bluest at 18,000 K? And if so, why did 35,000 K Lambda Orionis look so strikingly blue to me?

I can't say why things do or do not look certain colors to you. While it's possible your eyes are physiologically different from those of most people, I suspect it's simply how your brain processes color. Certainly, color perception is far more complex than just the chemistry of the retina would suggest. Different people certainly see color differently. I'll say that I've never seen a star that I'd call strikingly blue. In fact, I'd describe stars that are classified as blue as visually being cold white.

The reason why an 18,000 K blackbody is generally considered the most blue (and why most people see it that way) is that this is the temperature where you have the most "blue" (450 nm) energy in comparison with longer visual wavelengths. As you get hotter, the energy across the visual spectrum gets flatter, with relatively more energy at longer wavelengths compared to blue (you always have more in total at the short end, it's just that the distribution gets flatter).

I've been enjoying this discussion of color. It sort of brings things full circle to think that very hot stars putting out lots of ultraviolet radiation give us beautiful red emission nebulae!

[johnemac]

since the final phase of the planetary nebula phenomenon for the star itself
is collapse to the white dwarf stage (immediately following the generation of
the planetary nebula), how can the white dwarf be 10 billion years old, and
the nebula be only 20 thousand ?? am i missing something ?

[Chris Peterson]

since the final phase of the planetary nebula phenomenon for the star itself
is collapse to the white dwarf stage (immediately following the generation of
the planetary nebula), how can the white dwarf be 10 billion years old, and
the nebula be only 20 thousand ?? am i missing something ?

The star formed some 10 billion years ago. It's only been in its white dwarf phase a short time.

[starsurfer]

Starsurfer: Of course it shows it to be blue. After you do the color balancing, things which are more strongly emitting shorter wavelengths have nowhere else to go. And colors presented are most certainly influenced by saturation and sharpening, especially when a star is in the middle of a faint planetary nebula that someone is paying special attention to and possibly giving it selective treatment over the rest of the stars in the image. Anyway, I'm not saying that they aren't bluer, just that things are not necessarily blue. The stars in the middle of planetaries are definitely special stars.

For the star to appear blue after saturation, it must have been blue in the raw unprocessed data in the first place.

[geckzilla]

Starsurfer: Of course it shows it to be blue. After you do the color balancing, things which are more strongly emitting shorter wavelengths have nowhere else to go. And colors presented are most certainly influenced by saturation and sharpening, especially when a star is in the middle of a faint planetary nebula that someone is paying special attention to and possibly giving it selective treatment over the rest of the stars in the image. Anyway, I'm not saying that they aren't bluer, just that things are not necessarily blue. The stars in the middle of planetaries are definitely special stars.

For the star to appear blue after saturation, it must have been blue in the raw unprocessed data in the first place.

Incorrect. Raw => Color balance => Saturation increase
The color balance is where the blue is coming from.

[Chris Peterson]

For the star to appear blue after saturation, it must have been blue in the raw unprocessed data in the first place.

"Blue in the raw unprocessed data" isn't a meaningful concept. "Blue" is a physiological response. The raw data contains multiple channels each containing different spectral data, modified by filter widths, filter shapes, and sensor sensitivity.

From the image alone, we really don't have enough information to draw any accurate conclusions about the hue this star would appear given sufficient intensity (the color would change simply by changing the intensity).

What we do know, based on physical understanding, is that in the human colorspace a star this temperature should appear a cool white- that is, white with a higher intensity at the blue end of the spectrum. If there is any significant intervening dust (unlikely, but possible) the star could be slightly reddened- that is, it could drift towards a warmer white.

[starsurfer]

Starsurfer: Of course it shows it to be blue. After you do the color balancing, things which are more strongly emitting shorter wavelengths have nowhere else to go. And colors presented are most certainly influenced by saturation and sharpening, especially when a star is in the middle of a faint planetary nebula that someone is paying special attention to and possibly giving it selective treatment over the rest of the stars in the image. Anyway, I'm not saying that they aren't bluer, just that things are not necessarily blue. The stars in the middle of planetaries are definitely special stars.

For the star to appear blue after saturation, it must have been blue in the raw unprocessed data in the first place.

Incorrect. Raw => Color balance => Saturation increase
The color balance is where the blue is coming from.

When Don Goldman processes his images, the colour balance for the stars is based on a G2V star.

[Ann]

Starsurfer wrote:
When Don Goldman processes his images, the colour balance for the stars is based on a G2V star.

David Malin, too, used the color of the Sun as the definition of white for his color pictures.

Ann

[geckzilla]

Ok.

[Chris Peterson]

When Don Goldman processes his images, the colour balance for the stars is based on a G2V star.

Yes, but keep in mind that this still only leads to approximately accurate star colors. It also fails when you have a mixture of continuum and emission sources, since the nature of the filters used requires different color balance techniques on the two. So you end up doing a lot of masking or other isolation processes to separate stars from nebula, or you allow one or the other to have different colors.

Finally, it is critically important to keep in mind the difference between color and hue. Simply changing the brightness of an image changes all the colors. Most astronomical images have their saturation pushed in a way that shifts the colors outside of what our eyes would see, even if they were more sensitive. Accurate hues does not mean accurate colors.

[NGC3314]

The reason why an 18,000 K blackbody is generally considered the most blue (and why most people see it that way) is that this is the temperature where you have the most "blue" (450 nm) energy in comparison with longer visual wavelengths. As you get hotter, the energy across the visual spectrum gets flatter, with relatively more energy at longer wavelengths compared to blue (you always have more in total at the short end, it's just that the distribution gets flatter).

I have to differ. The form of the blackbody function shows that its slope across the visible (or any other range longward of the peak) saturates toward high temperatures, so that arbitrarily hot blackbodies are no redder than cooler ones. This plot uses the standard IDL astro library function Planck, and compares blackbodies at 10,000, 25,000, 50,000, and 100,000 K. They are normalized to the same observed flux at 10,000 Angstroms = 1 micron to show the optical slopes more clearly. This continues to steepen to shorter wavelengths (become more blue, if I take the shorthand astronomer's parlance and sidestep perceptual nuances), albeit with diminishing returns at high temperatures.

[starsurfer]

Not even a good comparison...my thinking....not the same type of object....Tarantula is not a Planetary Nebula....though further away by far...is more massive, contains more stars, is a total region in itself, etc....should really be compared to a Planetary Nebula of the same distance....

I think the comparison was meant to be of interest to imagers, who have some idea about the brightness of the Tarantula. The comparison was purely one of how many photons are received at the camera, nothing to do with the intrinsic brightness of the objects. So the types of objects aren't really important, only that they are extended and of similar scale.

A more useful comparison would have been between the separate Ha and OIII.